error with Static Library code

This is a discussion on error with Static Library code within the C++ Programming forums, part of the General Programming Boards category; hi all,
i'm learning the book "Teach Yourself Visual C++ 6 in 21 days" , and i'm in Chapter 16 ...

error with Static Library code

hi all,
i'm learning the book "Teach Yourself Visual C++ 6 in 21 days" , and i'm in Chapter 16 - Creating Your Own Classes and Modules . I did everything like it said but i've an error. This Static Library project has 2 classes , 1 class , called CLine , is added from other project and 1 class , called CModArt , is added in this project .
So this is the code to creat new line in CModArt class :

oh , i fixed this bug but i dont understand why . Because before that , I already have include "cline.h" in ModArt.cpp . But now when i also include "cline.h" in ModArt.h , it work gud?
so wat prob here cus i think i just need to include "cline.h" in ModArt.cpp , no need in ModArt.h ?